Tunable Unidirectional Guided Resonances via Multi-Singularity Interactions in Momentum Space

The topological nature of bound states in the continuum (BICs) has attracted significant interest in manipulating resonant states and light radiation through topological singularities in momentum space. In this work, we present a scheme to realize tunable unidirectional guided resonances (UGRs) based on interactions between multiple circularly polarized states (C points) in momentum space. We first demonstrate that C points can intrinsically exist around symmetry-protected BICs in photonic crystal slabs even without any symmetry reduction. Through controlled symmetry breaking, the BIC splits into a pair of C points, which can interact with the intrinsic ones. Their interactions lead to rich evolution processes of C points, including formation of two UGRs at different k points with opposite topological charges, and generation of two UGRs with identical charges accompanied by the annihilation and regeneration of C points. Furthermore, it is demonstrated that directional radiation with high asymmetry ratio can be achieved across a broad parameter range via the interactions between multiple C points. These findings suggest that the involvement of multiple singularities provides additional degrees of freedom for resonant mode manipulation, opening new avenues for investigating singularity dynamics in momentum space and applying unidirectional guided resonances in optical field control.